This invention relates generally to a sensor for sensing the location of objects buried in the loose sediment in the bed of the sea or other body of water, which loose sediment is called soil hereinafter.
Finding and identifying sea-bottom objects is a difficult problem, especially if the objects are partially or completely buried in the soil at the bottom of the sea. Long-range techniques using optics or sonar work in some cases, but optical techniques break down in turbid water, and optics and sonar are ineffective against buried objects. A long-range technique such as radar is useless in seawater, because seawater's high electrical conductivity causes excessive absorption of the field. However, a low-frequency electromagnetic field is sufficiently unattenuated in seawater that it offers some detection capability at limited ranges.
The classical techniques such as radar and sonar are appealing, because they allow the field of the source to be separated from the object's return field by convenient methods such as range gating to provide time separation. Optical techniques lend themselves to imaging. However, low-frequency electromagnetic techniques in seawater present formidable source-field/return-field separation problems. If an object to be detected is made of ferromagnetic material (iron or steel), the object will distort the ambient earth's magnetic field, and will be detectable at some range, depending on the object's size, by means of passive magnetic sensors. Aside from the problem of sensor sensitivity, the problem of moving the sensor about in the large ambient magnetic field of the earth precludes the use of vector magnetometers--they cannot be stabilized adequately, and therefore noise due to motion in the earth's magnetic field will obscure the anomalous signal. Two approaches which are currently under investigation use total field magnetometers, or carefully balanced tensor gradiometers which are relatively insensitive to the highly uniform earth's magnetic field. For sea-bottom buried objects composed of nonferromagnetic materials, low-frequency active electromagnetics appear to offer the only viable detection technique.
For active electromagnetic detection in seawater, the frequency typically is limited to the audio range. There are two principal reasons for this: (1) at 10 Khz the skin depth in seawater is approximately 2.5 meters, dropping to less than a meter at 100 kHz, and (2) for maximum sensitivity in a small volume, the sensor of choice is an induction magnetometer having a permeable core, and eddy current losses in the core become important as frequency increases.
In the audio frequency range, the displacement current in seawater is negligible compared to the conduction current, and the quasi-static approximation holds. Thus, the field equations are diffusive in character, and it is not possible to separate source field and return field by range gating in the conventional sense. If the detected object has high electrical conductivity, it is possible to use a pulsed magnetic coil source. Then, the current induced in the object has a smaller decay constant than that induced in the seawater volume, and the detected field shows a change in slope with time which, in principle, can be exploited for detection. However, the usefulness of this approach in the detection of small localized objects, as opposed to characterizing vertically layered conducting strata, is not at all established, and constitutes a separate research issue. In the work leading to the present invention, we have investigated active electromagnetic detection with a time-harmonic continuous-wave (CW) source.
The conventional low-frequency active electromagnetic detector consists of a driven coil carrying a stable, time-harmonic circuit, and a sensor coil which is made insensitive to the drive coil field by means of relative geometry, and electronic compensation techniques. Examples of compensation techniques include: (1) the use of relatively insensitive, intermediately positioned reference coils whose outputs are used to cancel the drive coil signal at the sensor coil by means of feedback, and (2) direct feedback of the drive coil signal to the sensor coil with rejection of a very narrow range of frequencies about the driven coil frequency. The latter technique requires relative motion between the detector and object in order to produce sufficient signal bandwidth for detection, and commonly is used in proximity fuzes. The driven-coil/sensor-coil arrangement has a number of features which limit its performance as a detector of sea-bottom buried objects.
The magnitude of the source magnetic field at the sensor coil is very large compared to the field due to the detected object. For this reason, the ability to reject the source magnetic field, not the sensor sensitivity, limits the performance of the detector.
The magnetic field of the driven coil induces magnetization in a permeable object and eddy currents in an electrically conducting object. Apart from attenuation and boundary effects, the source magnetic field obeys an inverse-cube power law in range. The object, in turn, develops an induced magnetic dipole moment, and the anomalous magnetic field also obeys an inverse-cube power law in range, giving a detection system whose sensitivity varies nominally as the inverse-sixth power of the separation distance between detector and object. This severely limits the range of the detector.
The driven coil magnetic field induces eddy currents directly in seawater, and these currents can interact weakly with the conductivity contrast presented by an object. The secondary magnetic fields produced by this process are small compared to those produced by the processes described above, and this type of detector is not very effective against non-conducting objects in seawater.
The presence of the seawater eddy currents will cause secondary fields to be seen in the sensor coil when the detector is near the sea bottom. This will cause the detector to be sensitive to its position and/or orientation relative to the sea bottom. This may be seen by visualizing orthogonal driven coils and sensor coil arrangements and the geometry of the sea-bottom image fields of the driven coil. The only configuration which does not see the bottom is one in which the sensor coil is on the driven-coil axis with its sense axis orthogonal, and both axes are parallel to the bottom. Even this combination will respond to the bottom if there is roll about the driven-coil axis.